1. Field of the Invention
The present invention relates to a servo motor control device for carrying out feedback control for a servo motor configured to drive a feed axis of a machine tool.
2. Description of Related Art
Operations of the feed axis of a machine tool driven by a servo motor include cutting-feed to move the feed axis of the machine tool while cutting a workpiece if the operating command in a machining program to machine the workpiece using the machine tool is a cutting-feed command, and a rapid-traverse operation to move the feed axis of the machine tool without cutting a workpiece if the operating command in a machining program to machine the workpiece using the machine tool is a rapid-traverse operation command. While priority is given to accurate cutting rather than quick movement to a destination in the cutting-feed, a quick movement to the destination is required in the rapid-traverse operation, and therefore, the feed speed and acceleration of the feed axis during the cutting-feed are set lower than the feed speed and acceleration of the feed axis during the rapid-traverse operation.
In a servo motor control device for carrying out feedback control for a servo motor configured to drive a feed axis of the machine tool, in order to control vibration of the servo motor, a servo motor control device, which uses an arithmetic coefficient value for the rapid-traverse operation and an arithmetic coefficient value for the cutting-feed higher than the arithmetic coefficient value for the rapid-traverse operation, as arithmetic coefficients, such as position control gain, proportional gain, integral gain, etc., for creating a velocity command and a torque command to the servo motor, is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-277994 (JP2000-277994A) and Japanese Unexamined Patent Publication (Kokai) No. 2003-216243 (JP2003-216243A). In this case, when the operation of the feed axis is switched from the rapid-traverse operation to the cutting-feed or from the cutting-feed to the rapid-traverse operation, the servo motor control device temporarily stops the servo motor and discontinuously changes the arithmetic coefficient from the arithmetic coefficient value for the rapid-traverse operation to the arithmetic coefficient value for the cutting-feed, or from the arithmetic coefficient value for the cutting-feed to the arithmetic coefficient value for the rapid-traverse operation.
In a servo motor control device, which discontinuously changes the arithmetic coefficient as described above, if the arithmetic coefficient is changed discontinuously without stopping the servo motor in order to reduce the machining time of the workpiece, the torque command becomes discontinuous caused by discontinuous velocity command, and therefore, there is a disadvantage that the shock of the servo motor increases.
On the other hand, as a control device for carrying out feedback control, a control device, which continuously changes arithmetic coefficients between two modes, is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-39061 (JP2000-39061A), Japanese Unexamined Patent Publication (Kokai) No. 2001-290501 (JP2001-290501A), Japanese Unexamined Patent Publication (Kokai) No. 2009-303423 (JP2009-303423A), and Japanese Patent No. 4441493 (JP4441493B). In the case where such a control device is applied to the servo motor control device and the operation of the feed axis is switched from the rapid-traverse operation to the cutting-feed or from the cutting-feed to the rapid-traverse operation, the arithmetic coefficients change continuously even if the servo motor is not stopped. Due to this, the velocity command changes continuously, and therefore, the torque command does not become discontinuous and it is possible to avoid an increase in the shock of the servo motor.
In the case where the operation of the feed axis is switched from the rapid-traverse operation to the cutting-feed in the control device, which changes arithmetic coefficients continuously between two modes, the change of the arithmetic coefficient is started at the time of switching from the rapid-traverse operation to the cutting-feed and when a predetermined time elapses from the start of the cutting-feed, the arithmetic coefficient reaches the arithmetic coefficient value for the cutting-feed. Due to this, during the period from the start of the cutting-feed to the time when the arithmetic coefficient reaches the arithmetic coefficient value for the cutting-feed, the arithmetic coefficient is lower than the arithmetic coefficient value for the cutting-feed, and therefore, the value of the torque command is low and there may be a case where the torque of the servo motor is not sufficient. Consequently, there may be a case where the machining accuracy of the machine tool deteriorates immediately after the feed axis of the machine tool is switched from the rapid-traverse operation to the cutting-feed.